How does the fac on Mix Shader work?
When you connect a BW image into the FAC input of the Mix Shader:

Black areas mean A
White areas mean B    



When using the FAC slider:
0 means A
1 means B  



Universal rule:
For all nodes (roughness, metalness, fac...)
Black means 0, white means 1.
You can use gradual transition as well.